1. Technical Field of the Invention
The present invention relates to a magnetic data processing device, a magnetic data processing method and a magnetic data processing program, and more particularly to a technology for determining the reliability of magnetic data.
2. Description of the Related Art
Conventionally, a magnetic sensor has been used to measure orientation or the like. When magnetism is measured using a magnetic sensor, the output of the magnetic sensor includes not only components of a measurement target such as Earth's magnetism but also noise or offset components. For example, in the case where Earth's magnetism is measured by a magnetic sensor in a Personal Navigation Device (PND) mounted in a vehicle, the offset component is generated by magnetization of the vehicle or the PND, or is generated by temperature characteristics of the magnetic sensor. The offset component varies as operating environments such as magnetization of the vehicle or temperature vary. Therefore, the output of the magnetic sensor is corrected by subtracting an offset component derived based on the output of the magnetic sensor from the output of the magnetic sensor (for example, see Patent References 1 and 2).    [Patent Reference 1] Japanese Patent Application Publication No. 2007-107921    [Patent Reference 2] Japanese Patent Application Publication No. 2007-139715
However, in the case where Earth's magnetism is measured by a magnetic sensor in a PND mounted in a vehicle, the offset component is caused by a magnetic field generated by an electronic circuit mounted in the vehicle or the PND, or is caused by fluctuations of an output of the magnetic sensor complying with Gaussian distribution. Accordingly, it is difficult to perform correction perfectly negating the noise component. Therefore, in a system using such a magnetic sensor, an error is included in data derived from the output of the magnetic sensor. However, when the output of the magnetic sensor contains a great error or when an error in one magnetic data element is amplified by an error in another magnetic data element, the reliability of the system is significantly reduced. Thus, it is important to determine the reliability of magnetic data output from the magnetic sensor.
Generally, the reliability of magnetic data is determined by comparing an index representing a distribution of the magnetic data with a given threshold.
However, there are the following problems if the reliability of magnetic data is determined using a preset and fixed threshold in the case where the magnitude of the noise component varies depending on the operating environment of the magnetic sensor. That is, in the case where the threshold is set for an environment in which the noise component is small on average, it is often not possible to use the magnetic data (i.e., the rate at which the magnetic data cannot be used is high) in an environment in which the noise component is great on average since it is often determined that the reliability of the magnetic data is unacceptable in the environment in which the noise component is great on average. On the other hand, in the case where the threshold is set for an environment in which the noise component is great on average, the reliability of the system is reduced in an environment in which the noise component is small on average since it is determined that the reliability of magnetic data is acceptable in the environment in which the noise component is small on average even when the reliability of magnetic data is low to the extent that the frequency of use of the magnetic data is not reduced below a practical range even if the magnetic data is determined to be unacceptable.